In recent years, mass spectrometry has become an important analysis tool in the physical and biological sciences. Mass spectrometry is an analytical technique that is used primarily to determine masses of particles, an elemental composition of a sample or the chemical structure of a molecule. Mass spectrometry works by creating ions from a sample to generate charged atoms, molecules or molecule fragments and measuring their mass-to-charge ratios.
In many implementations of mass spectrometry, to achieve the maximum possible sensitivity, ions created at higher pressures need to be transmitted with high efficiency through narrow, conductance limiting apertures that separate differentially pumped vacuum chambers prior to reaching the high vacuum region of the mass analyzer. In the mass analyzer, ions are sorted by their masses by applying electromagnetic fields. Thus, the sensitivity of the instrument is directly related to how efficiently ions are transmitted to the mass analyzer. The ion transmission efficiency depends on the extent to which the motion of ions can be controlled in the different vacuum stages.
In the absence of background gas molecules (e.g., high vacuum), ions can be manipulated with extreme precision and in a well understood fashion using magnetic and electric fields. At elevated pressures (e.g., about 1 Torr and above), collisions with gas molecules increasingly dominate the behavior of ion motion and it becomes much more challenging to control ion motion over larger areas or volumes. For example, the high rate of collisions inhibits effective focusing of ions with static lens stack. Further, radio frequency-only multipoles exhibit either an acceptance area that is too small to efficiently capture ions from an expanding gas jet (for small inscribed radius) or an effective potential that is too weak to focus ions to a narrow conductance-limiting aperture (for large inscribed radius).
One approach to solving this problem is to use a skimmer as a conductance-limiting orifice to separate the first and the second vacuum chambers. However, the use of a skimmer causes only a small fraction of the ion cloud to be sampled, which reduces the efficiency of the ion transmission and creates a major sensitivity bottleneck for mass spectrometry. Another approach to solving the problem is to use an ion funnel.
A traditional ion funnel uses a series of closely spaced ring electrodes whose inner diameters gradually decrease, serving to radially confine ions as they pass through the funnel. The rings are arranged in a non-overlapping manner along an axial line, coincident with the direction of ion travel, to form a conic or funnel shape. In operation, an out-of-phase radio frequency potentials are applied to adjacent electrodes, and a dc gradient is typically applied in the direction of the axis of the ion funnel to drive ions through the device.
Ion funnels have been successfully implemented to improve the sensitivity of many mass spectrometer designs. However, there are some mass spectrometry applications where a traditional ion funnel configuration is not optimal. In view of the above, new methods and apparatus for ion control using an ion funnel are desired.